Runoff is water from rain or melted snow that is not absorbed and held by the soil, but runs over the ground and through loose soil. As runoff moves, it picks up and carries pollution, which it can deposit into ponds, lakes, coastal waters, and underground sources of drinking water. Urban and industrial runoff is runoff from cities and suburbs where there are roads, parking lots, pavement, construction sites, rooftops, and industrial facilities. (Rural areas may also have concerns with agricultural runoff carrying animal waste and farm pollutants directly into waterways.) Many urban areas have stormwater drain systems that channel the runoff into local waterways through pipes, ditches, and drainage canals. Stormwater systems reduce flooding, mud, and erosion in urban areas. Runoff will also flow directly into local waterways.

Urban runoff is one of the leading sources of water pollution in streams, lakes, rivers, and reservoirs, according to a federal report. Pollution from urban and industrial runoff can include heavy metals, dirt, and salt from roads; pesticides and fertilizers from lawns and gardens; and oil, grease, antifreeze, and other toxic chemicals from vehicles and vehicle maintenance facilities. Runoff can include viruses and bacteria from pet waste and failing septic systems. It can contain heavy metals from vehicles, buildings, bridges, and industrial facilities; garbage; and other debris. Industrial runoff can include liquids from animal hides in tannery yards, wastewater from meatpacking facilities, dirt and chemicals from construction sites, and other site-specific pollutants.

The pollution in urban and industrial runoff can contaminate drinking water, harm or kill fish and wildlife, destroy marine habitat, and make recreational areas unsafe and unpleasant.

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